Activity trackers and sleep tracking apps will happily give you stats on your sleep: How many hours you were in the sack, for example, and whether that sleep was good “quality.” But you can’t take those numbers at face value, and some of them are flat out wrong.

Movement is not the same thing as sleep, Dr. Winter emphasizes, but sleep labs do monitor movement—along with other body functions, like breathing, eye movement, and brain activity.

So you can ignore the number FitBit gives you for “sleep efficiency,” or Sleep as Android’s opinion on how much time you spent in “light” versus “deep” sleep. There’s no way they could possibly have a complete picture of when you were truly asleep, or what stage of sleep you were in.

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It’s also normal to move during sleep, and to wake up a few times during the night. We usually don’t remember these movements or awakenings. So if you see them on your sleep tracker’s output, don’t worry. As long as you feel well-rested in the mornings, the movements aren’t a problem.

Sleep Trackers Are Great for Detecting Patterns Over Time

Where apps and trackers excel, on the other hand, is their ability to give you big picture data. “It’s very difficult to get 30 days of in-lab polysomnographic data,” says Dr. Winter, but easy to wear a FitBit or sleep next to your phone for that time.

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If you’re worried that you don’t get enough sleep, the tracker can help you figure out whether that’s true. Dr. Winter told us the story of a lawyer who was convinced she never slept more than an hour a night, but her FitBit recorded six hours. She thought the FitBit was broken. A lab study showed that it was correct: she was getting a lot more sleep than she realized.

On the other hand, Dr. Winter decided to track his own sleep, and found that he stayed up later than he thought he did. “It told me very bluntly, ‘Hey man, you’re telling everybody you’re getting 7 hours of sleep, you’re a fuckin’ liar, you’re only getting 6 hours and 15 minutes, on average.’ That’s not enough.” He now puts more effort into making sure he gets to bed on time, instead of getting distracted with late night tasks and TV.

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Sleep tracking can also help you spot patterns when something in your life changes: if you start a new job or exercise routine, the sleep tracker can help you make sure you’re still getting enough sleep. Similarly, some medications can make your sleep more restless, and your app will report more awakenings or movements during the night, so you can see if there’s a problem. If you end up feeling sleepy during the day, this data can help you track down the cause.

Experiment on Yourself and Focus on the Big Picture

Dr. Winter suggests one more benefit of sleep trackers: they let you test, for yourself, what really does and doesn’t affect your sleep.

But if you don’t have a particular question or concern about your sleep, it may be best to ignore your sleep data for now, and there’s little reason to go invest in a sleep tracker “just because.” Just because it’s possible to measure a number doesn’t mean it’s a number worth worrying about, or even one that’s medically useful. Use your sleep tracker as a tool to address problems or questions that you have, not as an all-knowing judge of whether you’re sleeping the “right” way.